Having been down that particular road, can perhaps offer some thoughts from my experience, that may or may not apply to you.

The first chemo session is usually the easiest and sometimes has no worse results than a night of to much indulgence or just an upset stomach. Each patient and chemo are different.

We have very limited control in this life. We can control our words, our actions,
our reactions.
Everything else is beyond our control. A little modesty goes a long way towards accepting that.

Under the current circumstances, simply being who you are, but being there, by her side matters more than all the words you may want to say. Actions take precedence here. Knowing she can count on you, to go through this with her, will matter more. Some men can not handle it, and go get drunk, go find someone for comfort, and so on.

Chemo need not be the end point, it may merely be a step on the road, to at present who knows what. Nothing at this step is necessarily foreordained. In time you will know what is happening, but the first day of chemo is rarely that day.

Good luck.